I tried everything under the sun to get my MDD FW800 to boot off of USB but it absolutely refused, it would start to load the OS installer and I would get the gray "Stop" sign each time. I tried it with a few different drives.

I finally borrowed an old FW drive enclosure from one of my friends, used one of the HDs I had tried in the USB enclosures, and it booted up in target disk mode no problem.

Apparently MDD FW800 systems can't be booted off of USB.